The present invention relates to improvements in surface conditions of a rolling sliding member which comes in rolling or sliding contact with its mating face during use such as bearing ring, rolling element and retainer constituting a rolling bearing or cam follower. More particularly, the present invention can exert a great effect particularly when used in an application which is subject to great load and hence smearing or seizing such as roll neck bearing for steel rolling mill and bearing for railway vehicle.
For the purpose of preventing rust or improving lubricating properties in the initial stage of operation, it is commonly practiced to provide a formed film of manganese phosphate on the surface of various mechanical parts made of an iron-based metal which makes displacement relative to the surface of its mating member during use, e.g., sliding surface thereof. However, formation not only causes the rise in surface roughness but also worsens the dimensional accuracy by the thickness of the formed film. The rise in surface roughness and the deterioration of dimensional accuracy cause not only the rise in torque or calorific value in the initial stage of operation but also the deterioration of rotary properties. Therefore, it is commonly practiced to control the kind, concentration and other factors of the forming solution to be used so that the formed film can be reduced in thickness and formed by dense grains.
The process for the production of a chemically formed film of manganese phosphate will be generally described hereinafter. In some detail, when a rolling sliding member made of an iron-based metal is dipped in an aqueous solution of manganese phosphate, the aqueous solution of manganese phosphate undergoes primary dissociation to produce free phosphoric acid that dissolves iron on the surface of the substrate metal of the rolling sliding member to decrease the hydrogen ion concentration on the metal surface. While the equilibrium of dissociation of the foregoing aqueous solution of manganese phosphate moves over the surface of the substrate metal constituting the foregoing rolling sliding member, an insoluble manganese phosphate crystal is deposited on the surface of the substrate metal.
The manganese phosphate crystal thus deposited on the surface of the substrate metal is manganese and iron. The particle diameter of the crystal and the thickness and roughness of the deposit are affected by the components of the compound. Accordingly, the particle diameter of the manganese phosphate crystal deposited on the surface of the substrate metal increases to increase the surface roughness of the deposit depending on the composition of the foregoing aqueous solution of manganese phosphate which is a forming solution. When the surface roughness of the deposit thus increases, the substantial contact area of the surface of the rolling sliding member with the surface of its mating member decreases. In addition, protrusion interference between crystal particles causes early peeling of the formed film during operation.
The surface of the substrate metal which has been exposed after the peeling of the formed film has a raised surface roughness because iron has been dissolved during the process of production of the formed film. Therefore, after the peeling of the formed film, minute protrusions present on the rough surface of the exposed iron-based metal undergo interference with each other (collision of protrusions present on the surface of a pair of members which make displacement relative to each other), causing early damage such as smearing on the metal surface that can result in serious damages such as seizing in some extreme cases.
Under these circumstances, JP-A-6-159371 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a technique which comprises controlling the composition and concentration of the forming solution to make the surface roughness of a formed substrate metal as large as twice or less that of an unformed substrate metal. In accordance with this technique, even after the formed film has been peeled to cause the substrate metal to be exposed, an oil film can be formed between the metal surfaces to prevent the occurrence of serious damages such as early peeling and seizing on the metal surface.
The inner ring, outer ring and rolling elements constituting a rolling bearing are rolling sliding members for which the present invention is intended. The provision of a formed film such as manganese phosphate film on the surface of the inner ring, outer ring and rolling elements has heretofore been practiced for rolling bearing for roll neck as shown in FIGS. 9 and 10. A roll for rolling a metal comprises a column portion called roll neck provided in the central part of the both axial end faces. The column portion is supported by a rolling bearing as shown in FIGS. 9 and 10 in such an arrangement that it can be freely rotated with respect to a fixed supporting device. The rotary bearing device for roll neck shown in FIGS. 9 and 10 will be firstly described. The roll neck 2 provided in the central part of the both axial end faces of the roll 1 is rotably supported inside the housing 3 by a double-row tapered roller bearing unit 4. The double-row tapered roller bearing unit 4 comprises a plurality of tapered rollers 9, 9 provided between inner ring races 6, 6 provided on the outer surface of inner rings 5, 5 externally fitted onto the roll neck 2 and outer ring races 8, 8 provided on the inner surface of outer rings 7a, 7b internally fitted into the housing 3.
The roll 1 which is ratably supported by the foregoing bearing device is rotated at a rate of from 1,000 to 1,800 min−1 (r.p.m.) during the operation of the rolling apparatus. Accordingly, it is necessary that the bearing device comprise a grease enclosed therein so that it can be lubricated during the operation of the rolling apparatus. At the same time, it is necessary that the metal material to be rolled be sprayed with cooling water to prevent the temperature rise accompanied by rolling. Therefore, the bearing device is provided with a sealing device for preventing the lubricant from leaking out as well as preventing foreign matters such as the cooling water from entering into the interior of the bearing device. As the rolling bearing for bearing the roll neck 2 there may be used a cylindrical roller bearing besides the tapered roller bearing as shown in the drawings.
Anyway, the rolling bearing for rotably bearing the roll neck 2 of the rolling machine is used in severe atmospheres such as high temperature, high load and varying load. Further, as the rolling machine operates, foreign matters such as the cooling water and rolling scale often enter into the interior of the rolling bearing for bearing roll neck to cause mislubrication. The life of the rolling bearing for bearing roll neck which is subject to such severe conditions during use is not the rolling fatigue life as seen in ordinary rolling bearings. It is affected by the fatigue accompanying surface damages due to mislubrication or sliding friction. In other words, the foregoing rolling bearing often reaches its life due to fatigue accompanying surface damages before reaching rolling fatigue life.
In order to prevent the reduction of the life of rolling bearing for bearing roll neck due to this cause, it has heretofore been practiced to properly design the material and shape of the various members constituting this rolling bearing or the composition of the lubricant to be enclosed in this rolling bearing. Further, it has heretofore been practiced to subject the surface of various members constituting the rolling bearing such as inner ring race 6, outer ring race 8 and rolling surface of tapered rollers 9, 9 to formation to form a manganese phosphate film or baked molybdenum disulfide film thereon.
The foregoing formation makes it possible to prevent rust of various members constituting the foregoing rolling bearing for bearing roll neck (inner ring 5, outer ring 7a, 7b, rolling elements such as tapered rollers 9, 9) and scoring on the portion at which the inner surface of the inner ring 5 engages the outer surface of the roll neck 2. In other words, by forming a formed film such as manganese phosphate film or baked molybdenum disulfide film on the surface of various members constituting a rolling bearing which is subject to severe conditions during use as mentioned above, so-called metal contact involving direct contact of substrate metal of the various members can be prevented. At the same time, these members can be prevented from being rust.
In accordance with the technique described in above cited JP-A-6-159371, the durability of a rolling sliding member made of an iron-based metal can be improved to some extent. More improvements have been desired to improve further the durability of the rolling sliding member.
For example, in the case of the rolling bearing forbearing roll neck as described above, by forming a formed film on the surface of various members constituting the rolling bearing, the damage and rust of the contact area can be prevented as mentioned above. However, the constituent members which have been merely subjected to formation cannot necessarily exert a sufficient effect. This reason will be described hereinafter. The foregoing formed film can be early peeled in a severe operating atmosphere. In accordance with the conventional treatment involving mere formation, the substrate metal covered by a formed film has a rough surface having a high surface roughness. This is because the surface of the substrate metal is eluted with a solvent during the process of production of the formed film by formation. When this rough surface is exposed with the peeling of the formed film, the effect of preventing rust is lost. Further, the rough surface makes drastic metal contact with its mating surface, causing drastic abrasion that results in damages such as smearing and seizing.